api [nfc]: Factor out TopicName.canonicalize

Author: gnprice

lib/api/model/model.dart

@@ -661,6 +661,9 @@ extension type const TopicName(String _value) implements String {
   /// The string this topic is identified by in the Zulip API.
   String get apiName => _value;
 
+  /// The key to use for "same topic as" comparisons.
+  String canonicalize() => apiName.toLowerCase();
+
   TopicName.fromJson(this._value);
 
   String toJson() => apiName;

lib/model/message_list.dart

@@ -352,7 +352,7 @@ mixin _MessageSequence {
 bool haveSameRecipient(Message prevMessage, Message message) {
   if (prevMessage is StreamMessage && message is StreamMessage) {
     if (prevMessage.streamId != message.streamId) return false;
-    if (prevMessage.topic.toLowerCase() != message.topic.toLowerCase()) return false;
+    if (prevMessage.topic.canonicalize() != message.topic.canonicalize()) return false;
   } else if (prevMessage is DmMessage && message is DmMessage) {
     if (!_equalIdSequences(prevMessage.allRecipientIds, message.allRecipientIds)) {
       return false;